The present invention relates to a receiver for an in-band signalling system based on the multifrequency transmission of switching and other information through the audio-frequency band, and especially to such a receiver for use in a multifrequency signalling system employed between telephone exchanges.
In an in-band signalling apparatus for telephone exchanges, one or more sets of sinusoidal signals having predetermined frequencies are transmitted from one exchange to another for signalling purposes. As a typical example, the No. 5 signalling system standardized in 1964 by CCITT is known. In this system, the so-called register signals representing an office number and a subscriber number of a called party are composed of two predetermined frequencies selected out of six frequencies arranged from 700 Hz to 1700 Hz at invervals of 200 Hz. In addition, at frequencies 2400 Hz and 2600 Hz in the No. 5 signalling system, the line signals exist to control an in-band signal receiver and a metering device in a called office to be triggered upon start and termination of communication. The line signals are generally called "supervisory signals", and are interchanged between the telephone exchanges in a predetermined sequence. Since details of the signals and the signalling procedure in the No. 5 signalling system are disclosed on pages 312 to 314 and 323 to 327 of "Green Book", Vol. VI-2 published in 1973 by The International Telecommunication Union, further description will be omitted here. In short, basic operations for detecting the multifrequency signals discussed above are to determine whether or not a received signal belongs to a group of predetermined multifrequency signals, and if so, what is meant by that signal. In the conventional system, such a receiver for the in-band signalling has a plurality of analog band-pass filters having the pass-band centers at the respective frequencies of sinusoidal waves, subsequent rectifier circuits, analog low-pass filters and threshold value circuits. However, it is indispensable that these circuits must be stable and be of high precision over a long period of time under various environmental conditions such as temperature variation, aging variation, etc. Consequently, a great deal of labor is required for the selection of part elements employed in the respective circuits, the adjustment after the circuit assembly and so on, and also the reduction of the production cost was limited.
The development of the LSI (Large Scale Integration) technique due to the advance of semiconductor techniques, and the rapid development of the digital technology supported by the former are replacing some of the circuits employed in communication apparatuses traditionally composed of analog circuits by digital circuits. This trend is due to the following reasons. In digital signal processing, the processed information is a digital quantity and the processing circuits are also digital, so that the manufacture of the apparatus which is stable and of high precision over a long period of time is possible, provided of course that the clock frequency is stable, and the adjustment process for the production of the apparatus which is indispensible in the case of the use of the analog circuits for constructing the aforementioned receiver can be omitted. Furthermore, enhancement of the productivity as well as the reduction in size and economy of the apparatus can be greatly expected owing to the circuit integration into an IC (integrated circuit) or an LSI.
A digital receiver for use in an in-band signalling system has been already reported in an article titled "An All Digital Telephone Signalling Module" published in the Proceedings of the National Electronics Conference, October 1973, and also in a article titled "Digital MF Receiver Using Discrete Fourier Transform" appearing in the IEEE Transactions on Communications, Vol. COM-21, pp. 1331-1335, December 1973 (Reference 1). In the former article, band-pass filter characteristics are obtained by means of a digital filter, while in the latter article, in place of the digital filter, Discrete Fourier Transformation is adopted. The features common to these digital signal receiving systems are that digital signal processing is based on the same sampling frequency used for encoding a voice signal and that various functions of analog circuits are almost directly replaced by those of digital circuits, and hence, the amount of multiplication, addition and subtraction in the signal processing is, as a whole, large, so that the increase in the apparatus size is unavoidable.